This invention relates to cameras with diaphragm devices employing electro- or magneto-optical elements whose transmittance is varied by application of an electrical or magnetic field, or so-called solid state diaphragm devices, and, more particularly to cameras having a solid state diaphragm device employing an electrochromie material.
The conventional cameras have generally employed mechanical type of diaphragm devices. The fact that the system for controlling the operation of such diaphragm device is necessarily of mechanical structure and that, in the case of cameras using interchangeable lenses, there is a need to provide a complicated coupling mechanism for selectively establishing an operative connection between the diaphragm device in the interchangeable lens mounting and a control mechanism therefor in the camera body, form the causes for an increase in the camera unit cost, an increase in the bulk and size of the camera, a limitation of the degree of freedom of the camera design, or the like.
Taking these factors in consideration, attemps have been made to employ an electro- or magneto-optical element whose transmittance can be varied with variation of the electrical or magnetic field applied thereto as the diaphragm of the camera, and a wide variety of so-called solid state diaphragm devices have been proposed. As the above-described electro- or magneto-optical element, mention may be made of a Kerr cell utilizing the double refraction phenomenon, a transparent ferroelectric substance (PLZT), liquid crystal and Faraday element. This element is used in combination with a polarizer and an analyser. Besides these, there are known electrochromie elements and cataphoresis elements.
These solid state diaphragm devices have many characteristic features which the conventional mechanical diaphragms completely lack. The first is that since the mechanical coupling connection is unnecessary, it is of small size and light weight. Therefore, the degree of freedom on the camera design is largely increased. The second is that the diaphragm operates without causing the production of mechanical shocks and an unpleasant sound resulting therefrom at all. The third is that only one electrical switch suffices to perform a previewing operation with the diaphragm closed down. Further since the solid state diaphragm device works with at least electrical circuit means, it is suited for use in the recent, greatly electronically designed cameras with a great merit.
The present invention contemplates the use of an electrochromie substance. The term "electrochromie substance" herein used is a general term for substances which, when supplied with current, produce colors, or are subjected to a color change, and which then when the direction of current flow is reversed, discolor by themselves or regain the original ones, as the electrons supplied induce an oxidation-reduction reaction.
The important characteristic feature of the electrochromic cell is in its memory function. That is, the electrochromic cell when having once been colored by the current supply continues to remain colored after the current supply is cut off. Therefore, if the electrochromic cell is used as the diaphragm of the single lens reflex camera, it becomes possible that the diaphragm, after having once been excited by the current supply before a shutter release is actuated, is maintained in an operative or closed down position during the exposure operation even when the current supply is cut off. Therefore, the use of the electrochromic cell in the diaphragm device contributes to a decrease in the amount of consumption of electrical power, and particularly in the case of making exposures with long shutter times, a great advantage can be expected.
However, in applying the electrochromic cell in the practical diaphragm device of the single lens reflex camera, the following problems arise.
That is, as in the prior art, the cameras of the type in which the electrical power supply is latched by the first stroke of the shutter button have to release the latching of the power supply in synchronism with the completion of the shutter operation and stop the power supply. The mere application of the electrochromic cell to the conventional camera will result in that the termination of the exposure is not followed by the reset of the diaphragm to the full open position, and the field of view of the finder is left darkened. This is a very large drawback.
With the camera of the type having the main switch of the battery provided in separation, when it happens that the above-described main switch is opened, for example, at a time during long exposure, the electrochromic cell is caused to continue closed down. This drawback also must be eliminated.